Electric Power Steering (EPS) systems typically use an electric motor and a method of torque control to provide steering assist. When using a Permanent Magnet Synchronous Motor (PMSM), a method called Field Oriented Control (FOC) may be utilized, which allows alternating current (AC) phase motor voltage and current signals in a stationary reference frame to be transformed into a synchronously rotating reference frame, commonly referred to as a d/q axis reference frame, where the motor voltages and currents become direct current (DC) quantities. FOC torque control is commonly implemented through a closed loop current control method that employs high performance current regulators that minimize the error between commanded and measured currents to achieve near-perfect current tracking. Thus, current control requires the motor currents to be measured, which is typically achieved by measuring the phase currents of the electric machine, which are then transformed into the synchronous frame via a Park Transform, for example, to perform the control in the synchronous reference frame.
When a large offset error occurs in a phase current measurement, the closed loop current control operating in the synchronous reference frame adjusts the motor voltage so that the measurement of motor current matches the command. However, since the measurement is incorrect, the actual motor currents are incorrect. The failure mode results in motor position dependent motor torque and current errors, which may be perceived as a large torque ripple at the motor shaft, and potentially larger than rated motor currents (for the hardware design). When the torque ripple caused by the phase current measurement offset error becomes large enough, it can produce motor torque in the opposite direction from the motor torque command. When used in EPS systems, failures which produce torque in a direction opposite to the desired motor torque command result in increased efforts required by an operator.
Since the sum of all three phase currents must always be equal to zero, most motor control systems typically only employ two shunts for measuring the currents. In many offset error detection schemes used in systems with two phase current measurements, a goal was to detect the presence of an offset error, since it was irrelevant which specific shunt had failed. Due to varying system requirements, three shunt measurement systems are also now being widely employed. In such systems, if an offset error occurs on only a single phase, it is possible to continue running the system in feedback control mode using current regulators with the remaining two accurate measurements, and thereby, improving the fault tolerance and reliability of the overall drive system.